One cross-border tunnel was discovered by chance a week ago. A bus drove over a sinkhole in a road running along the border fence near the San Ysidro crossing and investigators discovered a dirt-filled bucket inside.

Another tunnel was found Wednesday by crews intent on filling the first.

The discovery of such tunnels from Mexico is rising – nearly as many have been found in the past 3½ years as in the 11 years before.

But the increased numbers aren't the result of a concerted effort to ferret out those who would use underground passages to sneak drugs, people or who-knows-what into the country.

"It's come to a point where the agents are obligated to look for any evidence that something might be transpiring beneath them," said Angel Santa Ana, a Border Patrol spokesman.

But agents don't have Superman-like power to see what lies underground, and they don't have the technology, either, he said.

"There are several private companies who are developing this technology and are testing it," Santa Ana said. "For now, we have to resort to the agents in the field doing their jobs, being vigilant and looking for any kind of evidence above ground."

It will probably be at least two years before underground imaging technology is available, he said, and then there's the question of how to pay for it.

Another agency within the Department of Homeland Security has also sought out the tunnels – with imperfect results.

U.S. Immigration and Customs Enforcement has agents who investigate efforts to breach the border.

This year, the agency hired a geophysicist to look for tunnels in Calexico and identified an underground "anomaly." Digging turned up nothing more than a change in soil structure.

The technology to look for hollow spaces is imprecise, and instruments are thrown off by electrical currents, old stream beds and soil differences.

"We need better and more sophisticated technology," said Michael Unzueta, acting special agent in charge of the agency's San Diego office.

Informants told agents about a cavernous tunnel in Otay Mesa that was discovered in 2002 before it reached the United States.

But tips from informants can also lead agents astray, he said.

Recently, agents used a search warrant to look for a tunnel in Campo in East County, and turned up nothing, he said.

Agents from Immigration and Customs Enforcement are part of a tunnel task force with the Border Patrol and the Drug Enforcement Administration, he said.

"We get a lot of information and obviously we act on it," he said.

But that doesn't eliminate the single biggest problem with tunnels.

"People ask us why can't we find more of them," Unzueta said. "We can't see them."